1. Field of the Invention
The present invention relates to a gas charge container, an atom probe apparatus, and a method for analyzing a hydrogen position in a material. Particularly, the present invention relates to a technique which directly measures atoms of a sample one by one, thereby improving the functions of an atom probe apparatus which can investigate the atomic structure of a very minute region. According to this technique, since the existence position of hydrogen in a material which was conventionally difficult to observe can be directly observed, the technique can greatly contribute to development of materials with excellent hydrogen embrittlement resistance.
Priority is claimed on Japanese Patent Application No. 2008-164619 filed on Jun. 24, 2008 and Japanese Patent Application No. 2009-115426 filed on May 12, 2009, the contents of which are incorporated herein by reference.
2. Description of Related Art
As introduced in Text of “Nanotechnology for Supporting the Advancement of Steels”, Shiraishi Kinen Koza (SMS-ISIJ), 53•54, the Iron and Steel Institute of Japan, an atom probe apparatus can measure the space position and element species of constituent atoms of a sample with a high spatial resolution of a nanometer or less, and is used for, for example, structure analysis at the atomic level of a conductive material. The principle of the atom probe apparatus applies a high DC voltage and a high pulse voltage to a conductive sample which has been worked in the shape of a needle, carries out field evaporation of sample surface atoms sequentially by a high electric field formed on the surface of the needle, and analyzes generated ions by a detector. It is also possible to radiate a laser instead of a pulse voltage, thereby assisting in the field evaporation to analyze a nonconductive material. Since the flight time until an evaporated ion reaches the detector is determined depending on the mass of the ion, this measurement then allows the element species of the ion to be determined.
Various kinds of atom probe apparatuses have been already developed. A three-dimensional atom probe apparatus with the highest practicality can measure the arrival coordinates of an ion by a coordinates detector, thereby measuring not only the arrangement of the ion in the depth direction but an actual three-dimensional space position in the sample. Normally, a three-dimensional element position distribution in a sample can be visualized with spatial resolution at a level of lattice spacing by calculating the flight direction of an ion in consideration of the electric field distribution of a needle tip with respect to a measurement atom data set in which data on 100,000 or more atoms is collected.
Meanwhile, hydrogen in a steel material causes hydrogen embrittlement (delayed fracture or the like), and countermeasures for this have been taken for a long period of time. It is considered that the hydrogen which has entered the steel becomes a starting point (generation of a crack) of a fracture such that the hydrogen diffuses in the steel, moves and accumulates to a specific spot, and weakens the bonding force between atoms or the like. The hydrogen embrittlement is a problem in a number of metallic materials as well as steel. Although hydrogen in a steel material is a very important element in determining the reliability of the material, the hydrogen cannot be identified by direct observation. This results from a very high diffusion velocity of hydrogen, and a slight amount of solid solution. Therefore, in order to observe hydrogen in steel, conventionally, various methods or apparatuses have been developed.
Now, a method which is a most used method for investigating the amount of hydrogen which exists (trapped, dissolved, or the like) in steel is the thermal desorption method. Two kinds of methods, such as the thermal desorption—gas chromatography in argon (the thermal desorption analysis: TDA) and the thermal desorption—mass spectroscopy in vacuum (the thermal desorption spectrometry: TDS), are mainly used in this thermal desorption method. However, although the trapped energy and abundance of hydrogen can be estimated, both the methods cannot directly investigate the existence position of hydrogen.
Journal of Japan Institute of Metals, Volume 58, No. 12 (1994), and pp. 1380 to 1385 reports that hydrogen or deutrium in a steel material was observed by the secondary ion mass spectrometry (SIMS). However, the spatial resolution remains at about 2 μm. In steel materials containing a fine precipitate of 0.1 μm or less, though the fine precipitate is considered that the hydrogen trapping capacity is high, it was impossible to investigate in which portion hydrogen exists.
In the tritium radioautography described in CAMP-ISIJ, Vol. 14 (2001), and p. 645, the information on the existence position of hydrogen can be obtained, for example, by depositing silver on an emulsion applied to the surface of a steel material, or a dry plate, using radioactive rays emitted from tritium.
Additionally, in the hydrogen microprint method described in CAMP-ISIJ, Vol. 13 (2000), and pp. 1379-1381, it is possible to indirectly trap the hydrogen emitted from the surface of a steel material as a silver particle by a redox reaction with silver bromide.
However, since emulsion is used in all of them, there is no spatial resolution of 0.1 μm or less. Therefore, the determination where hydrogen exists in the steel material could not be made.
In contrast with such conventional techniques, a three-dimensional atom probe (3DAP) method has a high spatial resolution, is able to detect hydrogen in principle, and has the possibility to detect the existence position of hydrogen. Since the diffusion velocity of hydrogen is very high, a phenomenon occurs in which the hydrogen charged into a sample comes out from a trap site, and comes out from the surface of the sample only by being held at room temperature in a short time after charging. Since the sample is cooled to and measured at 100K or lower, 3DAP is advantageous in that escape of trapped hydrogen is prevented. However, even if a needle-shaped material is worked in advance and hydrogen is charged into this needle-shaped material, a phenomenon which the charged hydrogen diffuses in a short time which is taken until the needle-shaped material is introduces into the 3DAP apparatus, and escapes from the surface of the needle-shaped material occurs in a very short time. Therefore, analysis was difficult.
An apparatus and measuring method which detect a material defect on a sub-nanometer scale (to 0.2 nm) are disclosed in Japanese Unexamined Patent Application, First Publication No. H9-152410. In this method, the deutrium gas is introduced into an analysis container (analysis chamber) using the 3DAP method, deutrium is charged into a heated needle-shaped material (sample), and the resulting material is analyzed by an atom probe. The reason why the deutrium gas is used is that remaining hydrogen gas exists even within the container (chamber) which has been highly evacuated, and this remaining hydrogen gas will be post-ionized and detected, thereby hindering the distinction from the hydrogen which exists within the sample. However, even if the deutrium gas is used, since the deutrium gas is put into the container before analysis, the deutrium will remain even if evacuation is made, and completely the same phenomenon as hydrogen occurs, i.e., the remaining deutrium will be detected by post-ionization. As a result, the distinction from the deutrium charged into the sample will be hindered. Moreover, a low temperature of 100K or lower is normally required for atom probe measurement. In an ordinary apparatus, however, a long period of time of 20 minutes or more is required until the sample is cooled to this temperature. Hence, since the deutrium charged until the cooling escaped from the sample, observation of the existence position of hydrogen to be targeted was difficult.
In consideration of such a situation, a new apparatus and method which measures the existence position of hydrogen in a material at an atomic level are demanded.